Many electronic devices contain electromechanical motors, such as piezoelectric actuators. Piezoelectric actuators are driven using bursts of single-ended or differentially applied voltage pulses. As each pulse is applied to a piezoelectric material contained within the actuator, the piezoelectric material changes shape through contraction or expansion, thus creating motion using the mechanical parts of the system. Piezoelectric actuators are used in many consumer electronic devices such as digital cameras and mobile phones. For example, a piezoelectric actuator may be used to adjust the lens of a camera in operations such as focus or zoom. A piezoelectric actuator may also be used to provide haptic feedback to the touchscreen of an electronic device, such as a mobile phone.
With the evolution of features and functions on mobile handheld devices, consumers are expecting continuous auto-focus and higher video capture rates for high definition video capture. With higher auto-focus frame rates, the camera lens has to move more often than it did in the past, for example, 60 frames per second compared to 15 frames per second or less. When using piezoelectric actuators, there is an increase in the intensity in the audible noise generated as the piezoelectric actuator is required to start and stop more often. This undesirable audible noise may be picked up by the microphone on the mobile handheld device. The audible noise may be reduced using an expensive mechanical design which may negatively impact features of a mobile handheld device, such as the height profile, the location of the microphone and cost of the device. Therefore, a need exists for an electrical drive scheme that will reduce unwanted noise content and will not impact the size or the cost of the mobile handheld device.
As discussed above, a common problem with piezoelectric actuators is that audible noise is produced during starting and stopping of the mechanical movement. Mechanical vibrations may be present during the starting or stopping of a piezoelectric actuator because of a sudden change in acceleration. Due to the mechanical vibrations produced during starting and stopping operations of the mechanical movement, the mechanical parts may produce audible noise that is generally undesirable. Previous solutions have developed ways of gradually starting and stopping an electromechanical actuator. Among these is the amplitude modulation of pulse width modulated (PWM) pulses driven to the actuator and a PWM controlled constant current source. However, a PWM controlled constant current source does not allow the generation of some signal patterns, such as a square signal patterns. Square signal patterns are required to drive friction driven actuators in order to take advantage of mechanical resonance.
Further, amplitude modulation solutions limit the programmability and control over the amplitude, which is undesirable in situations that require constant amplitude or that require the programmability of the amplitude to control audible noise. Further, using amplitude modulation to control start and stop operations of a mechanical movement generally requires complex circuitry used to generate a variable amplitude from a supply voltage of a device. The use of additional complex circuitry increases the design time, cost and efficiency of such a system. Therefore, a need exists for an improved solution to reduce or control audible noise during start and stop operations while providing programmability and control over the amplitude, reducing displacement and using a reduced silicon area.